As one example of an information recording medium, there is a phase change type information recording medium including a recording film on which recording, erasure and rewriting of information are performed by optical means using laser light. The recording, erasure, and rewriting for the phase change type information recording medium are performed using the occurrence of reversible change in a phase change material between a crystal and an amorphous material in the recording film. To be more specific, generally, the recording of information is conducted by amorphizing part of the recording film to form a recording mark, and the erasure is conducted by crystallizing the recording mark. The amorphization is performed by heating the recording film to a temperature which is equal to or more than a melting point, and then rapidly cooling the same. On the other hand, the crystallization is performed by heating the recording film to a temperature which is equal to or more than a crystallization temperature and is equal to or less than the melting point or heating the same to a temperature which is equal to or more than the melting point, and then gradually cooling the same.
As one example of the phase change type information recording medium, there is an information recording medium such as a “Blu-ray Disc” currently commercialized. This “Blu-ray Disc” is used as an information recording medium corresponding to digital hi-vision broadcasting (High definition television broadcasting), and some of them have recording densities of 25 GB (one layer), and 50 GB (single-side double-layer), and a transfer rate of 36 Mbps (regular speed). As a material for a recording film in a regular-speed rewriting-type Blu-ray Disc, for example, a solid solution of GeTe and Sb2Te3 (refer to Patent Document 1) or a solid solution of GeTe and Bi2Te3 obtained by replacing Sb by Bi (refer to Patent Document 2) is used.
In the “Blu-ray Disc”, a sputtering method is typically used to form the above-described recording film, a reflection film, and a dielectric film. The phase change thin film formed by this sputtering method, in most cases, is in an amorphous state. Accordingly, in the “Blu-ray Disc”, before recording information, the entire recording film in the data region of the information recording medium needs to be crystallized. Processing for crystallizing the entire recording film in the amorphous state is referred to as initialization.
In the information recording medium, recently, shortening of a light beam for recording and reproduction has been advanced to process more information at a higher speed to accumulate a large amount of information. This leads to the use of laser light of 405 nm (400 to 410 nm) as in the foregoing “Blu-ray Disc”. Moreover, in order to reduce manufacturing cost, a reduction in time required for the initialization for crystallizing the entire recording film has been demanded. For this reason, a configuration is necessary, in which a wide range of the recording film can be initialized in a short time using the light beam in a large spot shape having a predetermined power. Accordingly, laser light having a wavelength of 810 nm (800 to 820 nm) which easily brings about high output is used as the light beam for initialization.
FIG. 9 is a schematic diagram showing a general configuration of a conventional initialization apparatus. As shown in FIG. 9, an information recording medium 209 is irradiated with a light beam A from an optical head 200, so that a recording film of the information recording medium 209 is initialized. In FIG. 9, in a light source 201 emitting the light beam A having a wavelength of 810 nm, an output power of the light source 201 is controlled by a laser drive circuit 208. The light beam A emitted from the light source 201 passes through a light path correction means 206 and a collimator lens 207, and is reflected in a beam splitter 203. The light beam A reflected in the beam splitter 203 passes through an objective lens 205, and reaches the information recording medium 209. The light beam A reflected in the recording film or the like of the information recording medium 209 again passes through the objective lens 205, and enters a focus error detector 210. At this time, an electric signal indicating the light beam A detected in the focus error detector 210 is inputted to a focus error signal generation circuit 211. The focus error signal generation circuit 211 forms a focus error signal, and outputs the focus error signal to a focus servo circuit 212. The focus servo circuit 212 drives a voice coil 204, based on the focus error signal to adjust a position of the objective lens 205. In this manner, the focus servo circuit 212 drives the voice coil 204 to perform the position adjustment of the objective lens 205, so that the light beam A is focused on a predetermined recording film of the information recording medium 209. The laser drive circuit 208 and the focus servo circuit 212 are controlled by a controller 213 (refer to Patent Document 3).
Patent Document 3 discloses an initialization method for an information recording medium including two information layers as an information layer having a recording film, in the initialization apparatus configured as described above.